Electron guns

ABSTRACT

In an electron gun for use in electron beam instruments such as electron microscopes and microanalysers the contamination that builds up on the control electrode (grid) is removed periodically and preferably automatically with the instrument out of use, by electron bombardment from a heater and possibly from the cathode itself, the control electrode being made some hundreds of volts positive with respect to the heater during this period. The heater may be one already present for heating the cathode. The bombardment may be in two thirty-second pulses applied immediately after the instrument is switched off.

llnited States Schaedler et al. 316/28 Meat [1 [111 3,795,837

Qhang Mar. 5, 19741- ELECTRON- GUNS t Prima Examiner-Ro Lake Phil h Y k[75] Inventor N Y C ang or town Assistant Examiner-James B. Mullins iAttorney, Agent, or Firm-Scrivener, Parker, Scriv- [73] Assignee:Cambridge Scientific Instruments I n r nd Cl rke Limited, Cambridge,England v [22] Filed: Mar. 24, 1972 [57] ABSTRAKIT [2]] Appl. No.:237,800 In an electron gun for use in electron beam instruments such aselectron microscopes and microanalysers the contamination that builds upon the control [30] Foreign Pnomy Dam electrode (grid) is removedperiodically and prefera- Mar. 25, 1971 Great Britain 7,966/71automatically with the instrument out of use, electron bombardment froma heater and possibly U-S Clo the cathode itself, the control electrodeg [51] 11 11. Cl.f made some hundreds of vohs i iv i respect to Fleld 0Semc the heater during period. The heater y be one already present forheating the cathode. The bom- [56] References C'ted bardment may be intwo thirty-second pulses applied UNITED STATES PATENTS immediately afterthe instrument is switched off.

2,774,645 12/1956 Batchelor 316/28 X 2,783J 5 Claims, 1 Drawing FigureELECTRON cons This invention relates to electron guns used forgenerating beams of electrons in apparatus such as electron microscopes,X-ray micro-analysers and electron beam machining equipment. Such gunsgenerally comprise an electron-emitting cathode and a control electrodein the form of a so-called grid or Wehnelt electrode, followed by one ormore anodes to form the beam. The normal thermionic directly orindirectly heated cathode has limited emissive power for a given sourcesize and considerable effort has been directed to developing sources ofelectrons of greater emitting power but small size. Attempts to increasethe brightness of atungsten hairpin type of directly heated thermioniccathode result in a short life and have the further drawback ofproducing an undesirable spread of electron energies.

Lafferty has proposed some years ago the use of Ianthanum hexaboride orother rare earth borides as the electron emitting material, but the highreactive nature of these materials at the elevated temperature give riseto severe practical problems. Broers has proposed an electron gun usinglanthanum hexaboride emitter by making a cathode in the form of asharp-pointed rod of lanthanum hexaboride carried on a cooled mountingand indirectly heated by a solenoid-like heating coil around it.However, in addition to requiring a much higher vacuum than theremainder of the electron beam column, which can be troublesome inapparatus that has to be frequently demounted for other purposes, thisheated lanthanum hexaboride cathode causes contamination of the controlelectrode by particles and decomposition products emitted from thecathode. Consequently although the life of a cathode itself may be manytimes that of a tungsten hairpin thermionic cathode, the electron gunstructure may still require demounting at time intervals shorter thanthe life time of the cathode for decontamination. Attempts have beenmade to overcome the problem by the insertion of a protective capbetween the cathode and the control electrode but with limited. success.

The aim of the invention is to reduce this problem of contamination andincrease the interval between downtimes. According to the invention itis now proposed that, with the instrument out of normal use, the controlelectrode is heated by electron beam bombardment from the heater coilachieved by a positive potential applied to the control electrode withrespect to the heater coil. The effect of this is to force off thecontrol electrode any particles or decomposition products that may havebuilt up on it arid to return them to the cathode. The potential appliedto the control electrode may be of the order of four or five hundredvolts positive with respect to the heater coil.

It will be understood that this cleaning operation can be performed atany time, and without'demountingthe parts or letting down the vacuum.Where the heating is done by the existing cathode heater coil nomodification at all is required to the gun and the only modificationrequired to the instrument is the provision of the necessary switchingto apply the potential and perhaps additional heating current to thecoil if required. According to a further feature of the invention it maybe arranged that the cleaning operation automatically comes into actionafter use of the instrument. For example when the instrument is switchedoff at the end of a days work the positive potential on the controlelectrode and the current to the heating coil are automatically applied,either for an indefinite period or preferably for a limited periodcontrolled by a timer. In a preferred arrangement the cleaning iscarried out in two or more successive pulses, each having a duration of,for example, 30 seconds, and separated by an interval of only a fewseconds.

The invention will now be further described by way of example withreference to the accompanying FIG- URE which illustratesdiagrammatically an electron gun equipped with switching means forcarrying out the invention.

The gun is of the kind proposed by Broers and described in British Pat.No. 1,150,516 (and the corresponding US. Pat. No. 3,462,635 and GermanPat. No. 1,589,974) comprising an elongated cathode member 1 with apointed end 2, made from a rare earth boride, preferably lanthanumhexaboride, and heated indirectly by a heating coil 3 wound around itbut out of mechanical contact with it. The heat is imparted to thecathode member partly by radiation from the coil and partly by electronbombardment of the cathode by electrons from the coil, produced by anapplied potential difference indicated diagrammatically at 4. The coilis supplied from a direct current source 5 of a few volts. The electronsemitted from the pointed end of the cathode are formed into a beam by acontrol electrode or so-called grid 6 which is a few hundred or thousandvolts negative with respect to the electrode, and then accelerated by ananode 7 which is several thousand or tens of thousand volts positivewith respect to the cathode. In practice it is usual for the anode to beat earth potential and for the control electrode and cathode to be at asubstantial negative potential (indicated diagrammatically at 8) withrespect to earth. The control electrode 6 is negative with respect tothe cathode l, as indicated diagrammatically at 9.

The whole gun assembly is in an enclosure (not shown) at high vacuum.

It is found that, during prolonged use of the instrument in which thegunforms the source of an electron beam, contamination builds up on thecontrol electrode 6. This contamination is found to be made up oflanthanum (but little boron) from the cathode l and tungsten from theheater coil 3, also some extraneous material.

In accordance with the present invention provision is made fordisconnecting the control electrode from its normal potential andconnecting it instead to the positive side of a source 10 of somehundreds of volts, for example between 400 and 500 volts, of which thenegative side is connected to the heater coil; at the same time one endof the heater coil, for example the positive end where the coil issupplied from a direct current source 5, is electrically connecteddirectly to the cathode, cutting out the potential difference 4. Thuswhen the switching operation according to the invention is performed,the control electrode 6 is made some hundred of volts positive withrespect to both the heater coil and the cathode. Consequently it isbombarded with electrons emitted from the heater and cathode, and theseclean off the contaminating particles. In practice we find that thevoltage should be at least volts for a worthwhile effect, and not morethan 1,000 volts. The switching can be provided by two-way switchesshown diagrammatically at 11 and 12, which are operated only when theapparatus is out of normal use, and when the high negative potential 8has been disconnected. In the drawing they are shown in their restpositions, i.e., those positions they occupy when the instrument is innormal use. At the same time a further two-way switch 13 connects theheater coil to a higher voltage supply 14 than the supply which feeds itduring normal use. Consequently the coil runs at a higher temperaturethan during normal use.

The switches ll, 12 and 13 are controlled automatically by a timer 15which is set in operation by the action of switching off the main hightension and heater coil supply of the instrument (electron microscope,micro-analyser or other equipment) in which the gun is employed, after aperiod of use. In the example under consideration the timer changes overthe switches l1, l2 and 13 for a period of 30 seconds and then, after .agap of a few seconds, for a further period of 30 seconds, and thenswitches itself off, returning the switches to their normal positions.It will be understood that the number, magnitude and duration of theswitchon pulses may be altered to suit a particular gun.

We claim:

1. In an electron gun comprising a heater, a cathode, a controlelectrode and at least one anode the improvement which comprises meansfor automatically removing contamination from said control electrodeafter each operation of said gun, said means comprises first switchingmeans operative automatically on switchingoff of said gun and acting toenergise said heater and second switching means acting on switching-offof said gun to apply a positive potential to said control electrode withrespect to said cathode, and timing means acting to de-activate saidfirst and second switching means after a limited period of time.

2. In an electron gun, the automatic contaminationremoval means setforth in claim 1 wherein said timing means are operative to activatesaid first and second switching means for a second limited period oftime following said first period after an interval.

3. In an electron gun the contamination-removal means set forth in claim1 wherein said first switching means are operative to apply a potentialto said control electrode which is positive with respect to said heater.

4. In an electron gun the contamination-removal means set forth in claim1 wherein said potential is of the order of 400 volts.

5. In an electron gun the contamination-removal means set forth in claim1 wherein said heater is a heater present for use in normal operation ofsaid gun and wherein said first switching means are operative toenergise said heater at a higher potential than that used in normaloperation of said gun.

1. In an electron gun comprising a heater, a cathode, a controlelectrode and at least one anode the improvement which comprises meansfor automatically removing contamination from said control electrodeafter each operation of said gun, said means comprises first switchingmeans operative automatically on switching-off of said gun and acting toenergise said heater and second switching means acting on switching-offof said gun to apply a positive potential to said control electrode withrespect to said cathode, and timing means acting to de-activate saidfirst and second switching means after a limited period of time.
 2. Inan electron gun, the automatic contamination-removal means set forth inclaim 1 wherein said timing means are operative to activate said firstand second switching means for a second limited period of time followingsaid first period after an interval.
 3. In an electron gun thecontamination-removal means set forth in claim 1 wherein said firstswitching means are operative to apply a potential to said controlelectrode which is positive with respect to said heater.
 4. In anelectron gun the contamination-removal means set forth in claim 1wherein said potential is of the order of 400 volts.
 5. In an electrongun the contamination-removal means set forth in claim 1 wherein saidheater is a heater present for use in normal operation of said gun andwherein said first switching means are operative to energise said heaterat a higher potential than that used in normal operation of said gun.